1) Field of the Invention
The present invention relates to an electronic imaging device, such as a digital camera or a video camera, and a portable terminal device including the electronic imaging device.
2) Description of the Related Art
Recently, the digital camera market has remarkably grown, and users' requirements for the digital camera have diversified. In particular, an improvement in image quality and a reduction in size are always in the users' requirements of great importance. Accordingly, both the improvement in performance and the reduction in size are also required for a zoom lens employed as a photographic lens.
One of the zoom lenses that meet such requirements is a small-sized zoom lens including a first lens group having a negative focal length and a second lens group having a positive focal length from an object side, and including a diaphragm on the object side of the second lens group (see, for example, Japanese Patent Application Laid-Open No. H10-039214).
In terms of the reduction in size, it is necessary to reduce an entire lens length (a length from a lens surface nearest to the object side to an image surface). In terms of the improvement in performance, the lens needs to have a resolution corresponding to at least an imaging element having three million to six million pixels over entire zoom areas.
Many users also desire a wide angle of view for the photographic lens. A half angle of view on a short focus end of the zoom lens is desired to be 38 degrees or more. The half angle of view of 38 degrees corresponds to a focal length of 28 millimeters in terms of a 35-millimeter film camera. Users further strongly demand a high magnifying power exceeding three.
One of the zoom lenses suited particularly for the requirement of the reduction in size includes a first lens group having a negative focal length, a second lens group having a positive focal length, and a third lens group having a positive focal length from an object side, including a diaphragm on the object side of the second lens group, and if the magnification is changed from the a short focus end to a long focus end, the second lens group monotonously moves from the image side to the object side, and the first lens group moves to correct a fluctuation of a position of the image surface following the magnification (see, for example, Japanese Patent Application Laid-Open No. H10-039214, Japanese Patent Application Laid-Open No. 2002-267930, Japanese Patent Application Laid-Open No. 2000-111798, Japanese Patent Application Laid-Open No. 2003-107348, and Japanese Patent Application Laid-Open No. 2003-35868).
As the zoom lens of this type, a high magnification zoom lens having magnifying power lens groups intended to realize a high magnification is also known (see, for example, the Japanese Patent Application Laid-Open No. 2002-267930).
As the zoom lens of this type, a high image quality zoom lens intended to realize an improved angle of view is further known (see, for example, Japanese Patent Application Laid-Open No. 2000-111798, Japanese Patent Application Laid-Open No. 2003-107348, and Japanese Patent Application Laid-Open No. 2003-35868).
Despite its basic configuration, each of these small-sized zoom lenses does not always include sufficient constituent requirements to realize the reduction in size.
As the zoom lens of this type, there is also known one which has a variable power ratio of about 2.5, which has a simplified lens configuration by using aspherical surfaces for respective lens groups, and which has a relatively wide angle of view (see, for example, Japanese Patent Application Laid-Open No. 2000-111798). However, because of its variable power ratio of about 2.5, this zoom lens does not satisfy the requirement of the higher variable power.
As the zoom lens of this type, there is further known one which employs lenses as many as six in a second lens group, and which has a high variable power ratio of 3.8 (see, for example, Japanese Patent Application Laid-Open No. 2003-35868). However, since a half angle of view of this zoom lens is only about 31 degrees, the zoom lens does not satisfy the requirement of the wider angle of view. Besides, since the six lenses are employed in the second lens group, the size of this zoom lens is disadvantageously, eventually increased.
As the zoom lens, there is also known one which has a variable power ratio of 2.5 or less, and which satisfactorily realizes the wide angle of view and the reduction in size by designing the configuration of a second lens group that is a variable power group (see, for example, Japanese Patent Application Laid-Open No. 2003-35868). However, since the variable power ratio of this zoom lens is 2.5 or less, the zoom lens does not satisfy the requirement of the higher variable power.
There is also known a zoom lens which has a variable power ratio of 3 or less, which has a sufficiently wide angle of view, and which satisfactorily corrects aberrations (see, for example, Japanese Patent Application Laid-Open No. 2003-107348). However, since the variable power ratio of the zoom lens is 3 or less, this lens does not satisfy the requirement of the higher variable power.
In each of the conventional zoom lenses, a maximum aperture area of the diaphragm is substantially constant irrespective of a zoom position. A minimum F number on the short focus end is about 2.4 to 2.8, whereas that on the long focus end is about 4 to 5.3. The difference between the short focus end and the long focus end in minimum F number is therefore great. An excessively small F number on the long focus end disadvantageously makes it difficult to correct the aberrations, and causes the deterioration in image quality. An excessively large F number on the long focus end disadvantageously causes the deterioration in resolution due to a diffraction phenomenon.